1. Technical Field
The present disclosure relates to a developing device, an image forming apparatus, and a process cartridge.
2. Description of Related Art
In electrophotography, two-component developing methods are widely employed that use a two-component developer comprised of toner particles and magnetic carrier particles. Two-component developing methods have an advantage over one-component developing methods in terms of durability and image quality. A typical two-component developing device includes a developer bearing member containing a magnetic field generator having multiple magnetic poles (hereinafter “developing sleeve”). The developing sleeve is configured to bear a developer on its surface and to convey the developer as it rotates.
Japanese Patent Application Publication No. 11-184249 describes a developing device having a developing sleeve containing a magnetic field generator. The magnetic field generator has five magnetic poles each of which is capable of generating a magnetic field having a strength enough for bearing a two-component developer. The five magnetic poles include a developer-supplying pole, a pre-developing developer-conveying pole, a developing pole, a developer-separating pole, and a post-developing developer-conveying pole. The developer-supplying pole contributes to supply of the two-component developer to the surface of the developing sleeve. The pre-developing developer-conveying pole contributes to conveyance of the supplied two-component developer to the developing area where the developing sleeve faces a latent image bearing member. The developing pole contributes to development of a latent image in the developing area. The developer-separating pole contributes to separation of the two-component developer from the developing sleeve after the developer has passed through the developing area. The post-developing developer-conveying pole is disposed between the developing pole and the developer-separating pole, and contributes to conveyance of the developer to the position where the developer separates from the developing sleeve after the developer has passed through the developing area.
A developer regulator is further disposed facing the developing sleeve between the developer-supplying pole and the pre-developing developer-conveying pole. The developer regulator is adapted to regulate the amount of two-component developer to be conveyed to the developing area. It is disclosed therein that the above configuration reliably performs the processes of supplying, conveying, and separating the two-component developer and developing latent images. Another two-component developing device has been also proposed further including a developer regulating pole disposed facing the developer regulator and no post-developing developer-conveying pole.
In accordance with recent demand for compact image forming apparatus, the developing device is required to be more compact, and therefore the developing sleeve is also required to have a smaller diameter. However, it may be difficult for a small-diameter developing sleeve to reliably perform the processes of supplying, conveying, and separating the two-component developer and developing latent images. This is because it is difficult for the small-diameter developing sleeve to contain at least five magnets which can generate a magnetic field having a strength enough for performing each process. Generally, the greater the magnetic force of a magnet, the greater the size of the magnet.
Japanese Patent Application Publication No. 2010-204639 describes a developing device including a magnetic field generator having only three magnetic poles which are capable of generating a magnetic field having a strength enough for bearing a two-component developer. The three magnetic poles include a developing pole, a pre-developing pole, and a post-developing pole. The developing pole generates a magnetic field in a developing area in which a developer bearing member faces an electrostatic latent image bearing member. The pre-developing pole generates a magnetic field for conveying a two-component developer supplied from a developer container to the developing area. The post-developing pole generates a magnetic field for separating the two-component developer from the developer bearing member at a downstream side from the developing area and an upstream side from the developing pole relative to a direction of rotation of the developer bearing member.
The above developing device is more compact because the magnetic field generator having only three magnetic poles occupies a much smaller space. Therefore, the developer bearing member can have a much smaller diameter. It is disclosed therein that the processes of supplying, conveying, and separating the two-component developer and developing latent images are reliably performed because such a compact developing device can generate a magnetic field having a strength enough for performing each process.
Such a compact developing device is likely to have a configuration such that the two-component developer is supplied from an upper side of the developer bearing member. The two-component developer supplied from the upper side of the developer bearing member is pressed against the developer bearing member due to its weight. The pressure from the two-component developer is different between an upstream side and a downstream side with respect to a supply screw that supplies the developer to the developer bearing member. At the upstream side, the two-component developer is pressed against the developer bearing member with a higher pressure and therefore the two-component developer forms dense ears on the developer bearing member. By contrast, at the downstream side, the two-component developer is pressed against the developer bearing member with a lower pressure and therefore the developer forms sparse ears on the developer bearing member. As a result, the resulting solid and halftone images may be lacking in uniformity between the upper side and the lower side with respect to the supply screw.